Choosing a Working Frequency

Author: Edmund A. Laport

The choice of working frequency is a compromise between efficient propagation, the necessary total portion of time that service must be maintained, and the technical complexities of operation involved. It requires skillful operation and coordination at transmitter and receiver to make frequent frequency changes without excessive lost time, but if this can be done and frequencies can be allocated for the purpose the very best propagation would be realizable.

Figure 3.6 is a sample diurnal frequency characteristic, more or less typical of tropical-belt conditions at a time near a sunspot maximum. It exemplifies clearly some basic ionosphere properties. Just before sunrise, the long period of darkness has permitted a gradual reduction in ion density by recombination of the molecules of the atmosphere in the ionospheric regions of the F layer. Therefore the maximum usable frequency falls gradually to a minimum. When the upper atmosphere is again subjected to sunlight bombardment, before sunrise is evident on the ground, ionization commences and maximum usable frequency shows a sharp rise. The rate of rise is very rapid, and this phenomenon on the maximum-usable-frequency and optimum-working-frequency curves is called the "sunrise wall" Ionization intensity (and accordingly the maximum usable frequency) rises to a maximum shortly after local noon at the place of wave reflection, following which it diminishes steadily into the presun-rise period.

FIG. 3.2. F2 zero - maximum usable frequency in megacycles W zone, predicted for January, 1047. (After Central Radio Propagation Laboratory).

Fig. 3.3. F2 4,000 - maximum usable frequency in megacycles W zone, predicted for January, 1947 (After Central Radio Propagation Laboratory).

FIG. 3.4. E layer 2,000 - maximum usable frequency in megacycles, predicted for January, 1947. (After Central Radio Propagation Laboratory.

FIG. 3.5. Median E sporadic, in megacycles, predicted for January, 1947. (After Central Radio Propagation Laboratory)

Fig. 3.6. Optimum working frequency for September, 1945, latitude 10 degrees north, west zone.

Every geophysical propagation path will have its own diurnal characteristics, and this changes month by month and year by year throughout the sunspot cycle. There will be some daily variations that are now predictable about 6 days in advance from direct sunspot observations, as these spots move into a certain part of the solar disk. These transient disturbances cause deviations from the typical diurnal frequency characteristic for any given path.

The propagation studies that must precede a choice of working frequencies can be made following the data and instructions issued by the Central Radio Propagation Laboratory. ) Since this is essential material for all modern high-frequency transmission, the reader should refer to the complete details and instructions in the application of the Central Radio Propagation Laboratory ionospheric data.

Another series of Central Radio Propagation Laboratory data () includes details of the various ionosphere heights. From these data the necessary information is available for determining the best angles of radiation in the vertical plane, depending upon the particular layer that is controlling as to frequency at any particular interval of time. These data are of special interest to the antenna engineer because the design of the antenna radiation patterns for best utilization of the medium proceeds from them. More extensive application of the frequency-height data permits the computation of multipath delays and angles of arrival of various wave groups from the different layers which are of importance in receiving antenna design.

The orientation of a wave path in the earth's magnetic field is another source of signal variability. Certain types of sunspots cause magnetic storms on the earth, and the effect on the ionosphere is greatest toward the magnetic poles, where the earth's magnetic field strength is greatest. It was found by Hallborg () that greatest disturbance to high-frequency transmission occurs when reflections take place from the ionosphere within or near the earth's auroral zones. Later researchers have established the boundaries of these zones more precisely, as shown in Fig. 3.7. It is therefore a rule of high-frequency propagation engineering to avoid all transmission through, or reflection within, these zones whenever possible. Where a transmission path must pass close to or into the auroral zone, much higher power is necessary to maintain any given circuit reliability.

The angular clearance of a wave path with respect to the boundaries of the auroral zone is called the "auroral-zone clearance" (AZC).

FIG. 3.7. Auroral-zone-absorption map (After Central Radio Propagation Laboratory)

The principles of multipath circuits are largely geometric, but the extent of layer penetrations at different angles of incidence and at different frequencies must be obtained from the characteristics of the ionosphere. Figure 3.8 is a simplified representation, on a flat-earth basis, of a circuit that can be worked with one hop, but there may be other higher-order hops present if there is a broad vertical lobe of radiant energy from the transmitting antenna. The angle of arrival for the one hop will be the lowest and will vary as the actual effective height of the controlling layer varies through the usual range. The E layer, when it exists, during the daylight hours, is at a relatively constant height and is shown as a single line.

FIG. 3.8. Flat-earth representation of a radio circuit from T to R by several possible wave paths.

The F₁ and F₂ layers are subject to considerable variation in height from time to time, and arbitrary upper and lower limits are shown by the two lines for each.

At a frequency that would penetrate E and F₁ with also some degree of reflection, there can arrive at the receiver three one-hop signals, one from each layer. The waves from the higher layers, having traversed the greater distance, will arrive at the receiver with increasing delays. Then consider two hops from each layer as shown, giving rise to three more wave groups with other time delays, the longest being that via F₂. Other higher-order hops might also exist.

One may eliminate E-layer reflections completely by choosing a frequency high enough to penetrate the E layer. If the frequency is close to the maximum usable frequency and F₂ is controlling, then F₁ reflections might be absent or very small. In such a case, there may be one-hop and two-hop F₂ waves, with their time differences of arrival. But if it happened that the large angle of incidence at F₂(B) and F₂(C), B and C being the reflection points on the F₂ layer, caused the waves at these points to penetrate without reflection also, only the one-hop F₂ signals would arrive. Under such conditions, only one wave would arrive completely free of multipath interference. At the other extreme, the operating frequency could be chosen so low that it would not penetrate the E layer at all, thus eliminating all F₁ and F₂ reflections. However, there may remain multipath E-layer signals unless the antenna radiation patterns at transmitting and receiving locations provide very low response at the higher angles at which multipath E-layer signals would be propagated.

The same situation exists when the example is changed into the spherical terrestrial geometry. The known methods of attack on the multipath problem involve these three factors: (1) the use of a frequency that will cause reflection from one layer only, as nearly as possible; (2) the use of a frequency near to the critical one for reflection at the point of lowest angle of incidence so that complete penetration occurs at other points in the same layer where the angle of incidence is higher; (3) the use of antenna directivities at both ends of the circuit that will focus the energy at the angle of one dominant wave group and discriminate against other multipath signals by relatively low response to all other angles.

High-frequency signals do not always follow a great-circle path. Some long circuits that have their terminals in northern and southern hemispheres, or in daylight and darkness, show large deviations from great circle in the angle of arrival of waves. The physics of this phenomenon are insufficiently understood at present, but it is evident that a wave will tend to follow a path of lowest propagational impedance when passing from one region to another of different propagational characteristics. The phenomenon is believed to be due in part to tilting of the ionized layers.

Angular deviations in azimuth and in the vertical plane can cause excessive signal variations at the receiver input if the antenna responses at both ends of the circuit change rapidly with the angle. The ideal antenna pattern is one that has uniform response over an angular sector in both horizontal and vertical planes and zero response at all other angles. Characteristics of this sort are not practically realized with present-day high-frequency antenna-design techniques and with present-day economics.